Alkali activated class C fly ash cement

Abstract

Class C fly ash may be activated with strong alkali to yield a novel, dense, quick-setting cement having unexpected properties. The setting time of the cement decreased as the pH of the activating solution increased. The microstructure and phase assemblage of the novel cement depended on the pH. Ettringite was absent beyond pH 14.30 (2.0N); a hexagonal plate-like crystalline form (str+E,uml a+EE tlingite (gehlenite hydrate, C2ASH8) and other compounds) became more abundant at higher pH. At higher pH the microstructure was characterized by high amounts of the plate-like crystalline phase and a dense matrix, due to higher reactivity of the glassy phase in fly ash. The novel cement will have uses in a number of areas, including fixation of hazardous wastes such as radioactive wastes, applications where rapid setting is desired, and formation of concrete in hot environments where ordinary Portland cements may tend to crack due to their heat of hydration.

We claim: 
     
       1. A cement which is denser than ordinary Portland cement, consisting essentially of stratlingite and calcium-silicate-hydrate, and either containing no ettringite, or containing ettringite in a concentration sufficiently small so as not to substantially weaken the dense microstructure of said cement as compared to the strength of the dense microstructure of an otherwise identical cement containing no ettringite. 
     
     
       2. A cement as recited in claim 1, additionally comprising unreacted Class C fly ash. 
     
     
       3. A cement which is denser than ordinary Portland cement, and which is the product of a reaction of an aqueous solution having a pH of at least 14.69 with a Class C fly ash, wherein: (a) the fly ash is present in an amount sufficient to produce a hardened solid following the reaction;   (b) the reaction mixture is either free of blast furnace slag and citric acid, or contains blast furnace slag or citric acid in a concentration sufficiently small so as not to substantially weaken the dense microstructure of said cement as compared to the strength of the dense microstructure of an otherwise identically prepared cement containing no blast furnace slag and no citric acid; and   (c) said cement is either free of ettringite, or contains ettringite in a concentration sufficiently small so as not to substantially weaken the dense microstructure of said cement as compared to the strength of the dense microstructure of an otherwise identical cement containing no ettringite.   
     
     
       4. A cement as recited in claim 3, wherein said cement has an initial setting time less than about five minutes. 
     
     
       5. A cement as recited in claim 3, wherein the reaction mixture additionally comprises a retarding agent, whereby said cement sets more slowly than it would set without the retarding agent. 
     
     
       6. A cement as recited in claim 3, wherein the reaction mixture additionally comprises a hazardous waste. 
     
     
       7. A cement as recited in claim 6, wherein the hazardous waste comprises a radioactive material. 
     
     
       8. A cement as recited in claim 6, wherein the hazardous waste comprises an alkaline radioactive material. 
     
     
       9. A cement as recited in claim 3, wherein the aqueous solution is prepared and cooled prior to the reaction. 
     
     
       10. A cement as recited in claim 3, wherein the aqueous solution is prepared and cooled to ambient temperature prior to the reaction.